The disclosure of Japanese Patent Application No. 2001-383142 filed on Dec. 17, 2001 including the specification, drawings and Abstract are incorporated herein by reference in its entirety.
1. Field of Invention
The present invention relates to an analog signal converter for converting digital signals supplied from a signal source into analog signals, and a mobile communication system using the analog signal converter.
2. Description of Related Art
In digital communication systems such as a code division multiple access (CDMA) system, a time division multiple access (TDMA) system, and a frequency division multiple access (FDMA) system, a digital signal is modulated and the modulated digital signal is then converted into an analog signal as an output signal of baseband circuits at the baseband final stage. Particularly, in the CDMA system which requires high-precision modulation, as shown in FIG. 11, it is well-known that a digital filter 14 is used and digital signals are converted into analog signals with oversampling techniques (for example, IS-95, 3GPP (W-CDMA) or the like).
When an analog signal converter 40 shown in FIG. 11 is used in the mobile communication system, for example, cellular phones, signals to be transmitted are modulated into signals within a frequency band used in radio communication by a digital modulator 12. Signals of a predetermined frequency band are extracted through the digital filter 14. The signals are supplied to a digital-analog converter (DAC) 20 through signal lines D and are then sampled using sampling clock CK. The sampled digital signals are converted into analog signals. The converted analog signals are then supplied to a radio-frequency (RF) circuit through a smoothing filter 22.
In the analog signal converter 40 shown in FIG. 11, the DAC 20 and the smoothing filter 22 located on the right side are analog circuits. The digital modulator 12 and the digital filter 14 on the left side are digital circuits. Therefore, the analog and digital circuits are generally designed as different functional blocks. The analog block is connected to the digital block by the signal lines D for transferring digital signals and a signal line for transferring sampling clock CK to sample the digital signals supplied to the DAC 20 through the signal lines D.
If the analog signal converter 40 is composed of an LSI chip mounting the DAC 20 and the smoothing filter 22 and an LSI chip mounting the digital modulator 12 and the digital filter 14, the power consumption in driving the signal lines D which connect both the blocks is not negligible. Where both the functional blocks are mounted on the same LSI chip, if they are arranged at a long distance from each other in the chip, similarly, the power consumption in driving the signal lines D is not negligible.
For example, in case of two LSIs, when it is assumed that a frequency of the sampling clock CK is 16 MHz, the amplitude of each signal line D is 3 V and 20 signal lines D exist in total, and the capacitance per signal line is 20 pF, power consumed by wiring capacitances of the signal lines amounts to about 14 mW. When it is assumed that power consumed by an output driver of the digital filter is about 30 mW, the total power consumption is about 44 mW. Thus, the analog signal converter disadvantageously consumes considerable power in the mobile communication.
Accordingly, it is an object of the present invention to overcome the above-mentioned disadvantages of the related arts. It is an object of the present invention to provide an analog signal converter which can reduce power consumed at the time when digital signals output from a digital circuit are transferred to an analog circuit and a mobile communication system using the analog-signal converter.
According to the present invention, an analog signal converter including an encoding circuit that sequentially encodes digital signals supplied in time series, a decoding circuit that sequentially decodes the digital signals encoded by the encoding circuit and a digital-analog converter that sequentially converts the digital signals decoded by the decoding circuit into analog signals.
Preferably, the encoding circuit obtains a difference between adjacent digital signals supplied in time series.
Preferably, the encoding circuit obtains a difference between adjacent digital signals supplied in time series and further obtains a difference between the differences.
Preferably, the encoding circuit does not encode one digital signal of the digital signals supplied in time series every predetermined number of digital signals and outputs the digital signal as is.
Preferably, the encoding circuit does not encode a digital signal at a time just prior to a positive or negative of the difference being inverted and outputs the digital signal as is.
According to the present invention, an analog signal converter for converting digital signals into analog signals has digital circuitry and analog circuitry and includes a digital modulator that modulates digital signals into modulated digital signals in a predetermined frequency band used in communication, a digital filter that extracts filtered digital signals in the predetermined frequency band from the modulated digital signals and an encoding circuit that sequentially encodes the filtered digital signals output from the digital filter. Moreover, the converter includes a decoding circuit that sequentially decodes the encoded digital signals and a digital-analog converter that sequentially converts the decoded digital signals into analog signals.
Preferably, the encoding circuit performs encoding by using sampling clock, or clock synchronously to the sampling clock, so as to sample the decoded digital signals in the digital-analog converter.
Preferably, the decoding circuit performs decoding by using sampling clock, or clock synchronously to the sampling clock, so as to sample the decoded digital signals in the digital-analog converter.
Preferably, the encoding circuit includes a memory for sequentially storing digital signals output from the digital filter, and a circuit for obtaining a difference between one digital signal and the next digital signal.
Preferably, the decoding circuit includes a memory for storing encoded digital signals, and a circuit for adding one encoded digital signal and the next encoded digital signal.
According to the present invention, there is provided a mobile communication system using an analog signal converter for sequentially encoding digital signals supplied in time series, sequentially decoding the encoded digital signals, and sequentially converting the decoded digital signals into analog signals.